Enhanced lift-off techniques for use when fabricating light sensors including dielectric optical coating filters

ABSTRACT

Light sensors including dielectric optical coatings to shape their spectral responses, and methods for fabricating such light sensors in a manner that accelerates lift-off processes and increases process margins, are described herein. In certain embodiments, a short duration soft bake is performed. Alternatively, or additionally, temperature cycling is performed. Alternatively, or additionally, photolithography is performed using a photomask that includes one or more dummy corners, dummy islands and/or dummy rings. Each of the aforementioned embodiments form and/or increase a number of micro-cracks in the dielectric optical coating not covering the photodetector sensor region, thereby enabling an accelerated lift-off process and an increased process margin. Alternatively, or additionally, a portion of the photomask can include chamfered corners so that the dielectric optical coating includes chamfered corners, which improves the thermal reliability of the dielectric optical coating.

PRIORITY CLAIMS

This application claims priority under 35 U.S.C. 119(e) to U.S.Provisional Patent Application No. 61/613,283, filed Mar. 20, 2012,which is incorporated herein by reference.

RELATED APPLICATION

This application is related to commonly assigned and commonly inventedU.S. patent application Ser. No. 13/______, filed the same day as thepresent application, which is entitled ENHANCED LIFT-OFF TECHNIQUES FORUSE WITH DIELECTRIC OPTICAL COATINGS AND LIGHT SENSORS PRODUCEDTHEREFROM (Attorney Docket No. ELAN-01287US0), which is incorporatedherein by reference.

BACKGROUND

FIG. 1 shows a cross section of an exemplary conventional light sensor102, which is essentially a single photodiode, also referred to as aphotodetector. The photodetector 102 includes an N⁺ region 104, which isheavily doped, and a P⁻ region 106 (which can be a P⁻ epitaxial region),which is lightly doped. All of the above is likely formed on a P⁺ or P⁺⁺substrate 108, which is heavily doped. It is noted that FIG. 1 and theremaining FIGS. are not drawn to scale.

Still referring to FIG. 1, the N⁺ region 104 and P⁻ region 106 form a PNjunction, and more specifically, a N⁺/P⁻ junction. This PN junction isreversed biased, e.g., using a voltage source (not shown), which causesa depletion region 110 around the PN junction. When light 112 isincident on the photodetector 102 (and more specifically on the N⁺region 104), electron-hole pairs are produced in and near the diodedepletion region 110. Electrons are immediately pulled toward N⁺ region104, while holes get pushed down toward P⁻ region 106. These electrons(also referred to as carriers) are captured in N⁺ region 104 and producea measurable photocurrent, which can be detected, e.g., using a currentdetector (not shown). This photocurrent is indicative of the intensityof the light 112, thereby enabling the photodetector to be used as alight sensor. The portion of the photodetector 102 that produces aphotocurrent in response to light incident on the photodetector can bereferred to as the photodetector sensor region, or simply as the sensorregion.

Photodetectors, such as but not limited to the exemplary photodetector102, can be used as ambient light sensors (ALSs), e.g., for use asenergy saving light sensors for displays, for controlling backlightingin portable devices such as mobile phones and laptop computers, and forvarious other types of light level measurement and management. For morespecific examples, ambient light sensors can be used to reduce overalldisplay-system power consumption and to increase Liquid Crystal Display(LCD) lifespan by detecting bright and dim ambient light conditions as ameans of controlling display and/or keypad backlighting. Without ambientlight sensors, LCD display backlighting control is typically donemanually whereby users will increase the intensity of the LCD as theambient environment becomes brighter. With the use of ambient lightsensors, users can adjust the LCD brightness to their preference, and asthe ambient environment changes, the display brightness adjusts to makethe display appear uniform at the same perceived level; this results inbattery life being extended, user eye strain being reduced, and LCDlifespan being extended. Similarly, without ambient light sensors,control of the keypad backlight is very much dependent on the user andsoftware. For example, keypad backlight can be turned on for 10 secondsby a trigger which can be triggered by pressing the keypad, or a timer.With the use of ambient light sensors, keypad backlighting can be turnedon only when the ambient environment is dim, which will result in longerbattery life. In order to achieve better ambient light sensing, ambientlight sensors preferably have a spectral response close to the human eyeresponse and have excellent infrared (IR) noise suppression. Such aspectral response is often referred to as a “true human eye response” ora “photopic response”.

FIG. 2 shows an exemplary spectral response of a photodetector (e.g.,the photodetector 102) without any spectral response shaping, e.g.,using a filter covering the detector. FIG. 3 illustrates the spectralresponse of a typical human eye (also known as the “true human eyeresponse” or the “photopic response”, as mentioned above). As can beappreciated from FIGS. 2 and 3, a potential problem with using aphotodetector as an ambient light sensor is that it detects both visiblelight and non-visible light, such as infrared (IR) light, which startsat about 700 nm. By contrast, notice from FIG. 3 that the human eye doesnot detect IR light. Thus, the response of a photodetector cansignificantly differ from the response of a human eye, especially whenthe light is produced by an incandescent light, which produces largeamounts of IR light. This would provide for significantly less thanoptimal adjustments if the photodetector were used as an ambient lightsensor, e.g., for adjusting backlighting, or the like. Accordingly,various techniques have been attempted to provide light sensors thathave a spectral response closer to that of a human eye, so that suchlight sensors can be used, e.g., for appropriately adjusting thebacklighting of displays, or the like. Some of these techniques involvecovering such light sensor with optical filters.

Typically, organic based optical filters cannot be used to provide atrue human eye response, because organic based optical filters do notsufficiently absorb and/or reflect infrared light. Rather, non-organicfilters, such as filters made of dielectric optical coatings, aregenerally preferred because they provide better performance. Suchdielectric optical coatings, which are made from stacks of variousdielectric films, are conventionally expensive to implement. This is inpart because they are typically patterned using a photoresist lift-offin a chemical solvent bath, which is typically costly due to therelatively long residence time (i.e., soak duration) in the photoresistsolvent bath, and due to the relatively narrow process margin.Alternatively, acoustic cleaning can be used, which is also typicallycostly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of an exemplary conventional photodetector.

FIG. 2 shows an exemplary spectral response of a photodetector, such asthe photodetector of FIG. 1, without any spectral response shaping.

FIG. 3 illustrates the spectral response of a typical human eye.

FIG. 4 illustrates a conventional lift-off process with a negativeprofile photoresist.

FIG. 5 illustrates a conventional lift-off process with a positiveprofile photoresist.

FIG. 6 illustrates a soft bake process, according to an embodiment ofthe present invention, which provides for a more rapid lift-off processand an increased process margin.

FIG. 7 provides illustrations of the photoresist reflow that occursduring the soft bake process.

FIG. 8 provides illustrations of the micro-cracks that occur in thedielectric optical coating as a result of the soft bake process.

FIG. 9 illustrates a thermal shock process, according to an embodimentof the present invention, which provides for a more rapid lift-offprocess and an increased process margin.

FIG. 10 illustrates an example of the temperature cycling involved inthe thermal shock process.

FIG. 11 illustrates a top down view of an exemplary light sensor beingmanufactured, wherein the light sensor includes a rectangularphotodetector sensor region surrounded by photoresist and covered by adielectric optical coating.

FIG. 12 includes perspective cross-sectional views that correspond to aportion of the light sensor shown in FIG. 11 during different steps of afabrication process.

FIG. 13, which is used to describe an embodiment of the presentinvention, illustrates a top down view of an exemplary light sensorbeing manufactured, wherein the light sensor includes a rectangularphotodetector sensor region surrounded by photoresist and covered by adielectric optical coating, and wherein the photoresist includesadditional corners compared to FIG. 11.

FIG. 14 includes perspective cross-sectional views that correspond to aportion of the light sensor shown in FIG. 13 during different steps of afabrication process.

FIG. 15, which is used to describe an embodiment of the presentinvention, illustrates a top down view of an exemplary light sensorbeing manufactured, wherein the light sensor includes a rectangularphotodetector sensor region surrounded by photoresist and covered by adielectric optical coating, and wherein the photoresist includes dummyislands that provide additional corners compared to FIG. 11.

FIG. 16 includes perspective cross-sectional views that correspond to aportion of the light sensor shown in FIG. 15 during different steps of afabrication process.

FIG. 17 is used to describe an embodiment of the present invention thatcombines the embodiment described with reference to FIGS. 13 and 14,with the embodiment described with reference to FIGS. 15 and 16.

FIG. 18, which is used to describe an embodiment of the presentinvention, illustrates a top down view of an exemplary light sensorbeing manufactured, wherein the light sensor includes a rectangularphotodetector sensor region surrounded by photoresist and covered by adielectric optical coating, and wherein the photoresist includes a dummyring that provides additional corners compared to FIG. 11.

FIG. 19 includes perspective cross-sectional views that correspond to aportion of the light sensor shown in FIG. 18 during different steps of afabrication process of an embodiment of the present invention.

FIG. 20 is a high level flow diagram used to summarize the soft bakeembodiments described with reference to FIGS. 6-8.

FIG. 21 is a high level flow diagram used to summarize the temperaturecycling embodiments described with reference to FIGS. 9-10.

FIG. 22 is a high level flow diagram used to summarize the embodimentsdescribed with reference to FIGS. 11-19.

FIGS. 23 and 24 illustrates top down views of exemplary light sensorsbeing manufactured, wherein the light sensor includes a rectangularphotodetector sensor region surrounded by photoresist and covered by adielectric optical coating having chamfered corners.

FIG. 25 is a high level flow diagram used to summarize the embodimentsdescribed above with reference to FIGS. 23 and 25.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments. It is to beunderstood that other embodiments may be utilized and that mechanicaland electrical changes may be made. The following detailed descriptionis, therefore, not to be taken in a limiting sense. In the descriptionthat follows, like numerals or reference designators will be used torefer to like parts or elements throughout. In addition, the first digitof a reference number identifies the drawing in which the referencenumber first appears.

Certain embodiments of the present invention relate to improvinglift-off processes used to produce light sensors (e.g., ambient lightsensors) with increased process margins. Accordingly, before describingsuch embodiments of the present invention, it is first useful todescribe conventional lift-off processes, so that the deficiencies beingovercome by embodiments of the present invention can be betterunderstood. Accordingly, conventional lift-off processes will first bedescribed with reference to FIGS. 4 and 5. More specifically, FIG. 4,which includes FIGS. 4( a), (b), (c) and (d), illustrates a conventionallift-off process that utilizes a photoresist having a negative profile(which is also known as a re-entrant profile). FIG. 5, which includesFIGS. 5( a), (b), (c) and (d), illustrates a conventional lift-offprocess that utilizes a photoresist having a positive profile (which isalso known as a tapered profile). The light sensors generally shown inFIGS. 4 and 5, and other FIGS., can have a structure similar to thephotodetector 102 described with reference to FIG. 1, but are notlimited thereto, as would be clear to one of ordinary skill in the art.For example, the light sensors can include P⁺/N⁻ junctions, or N⁺/P⁻junctions, or PIN, NPN, PNP or NIP junctions, but are not limitedthereto. Regardless of the exact structure of the light sensor, thelight sensor will includes a photodetector sensor region that produces aphotocurrent in response to light incident on the light sensor.

Referring to FIGS. 4( a) and 5(a), a photodetector sensor region 404,504 formed in a semiconductor substrate 406, 506 is covered with aphotoresist layer 420, 520 (also simply referred to as a photoresist).The photoresist 420, 520 is covered with a photomask 430, 530 and isexposed to ultraviolet (UV) light. As the term is used herein, UV lightis also meant to also include deep ultraviolet light (DUV). This causesexposure of a portion of the photoresist to the UV light, while anotherportion of the photoresist is unexposed to the UV light. Thereafter, adeveloper is used to remove the photoresist that was unexposed to UVlight, without removing the photoresist that was exposed to UV light, asshown in FIGS. 4( b) and 5(b). Depending upon the type of photoresistused, the post-develop photoresist can have a negative profile (alsoknown as a re-entrant profile), as shown in FIG. 4( b), or a positiveprofile (also known as a tapered profile), as shown in FIG. 5( b).Thereafter, a dielectric optical coating 440, 540 is deposited, tothereby form a dielectric optical filter over the light sensor, as shownin FIGS. 4( c) and 5(c). The dielectric optical coating, which can alsobe referred to as a dielectric optical filter, is made from a stack ofdielectric films. A lift-off process using a chemical solvent isthereafter used to remove the remaining photoresist 420, 520 and thedielectric optical coating 440, 540 covering the photoresist, resultingin the structures shown in FIGS. 4( d) and 5(d).

A problem with conventional lift-off techniques is that following thedielectric optical coating deposition (illustrated in FIGS. 4( c) and5(c)), the lift-off process requires a very long soak duration (on theorder of multiple hours) due to the photoresist not being fully exposedto the solvent. While the exact soak duration (also know as lift-offduration) depends on the thickness of the dielectric optical coating,the thickness of the photoresist, the chemical solvent, and the type oflift-off equipment used, tests have shown that soak durations of up tonine hours may be necessary to perform the lift-off process. Such highsoak durations are necessary, at least in part due to the dielectricoptical coating being highly conformal, even where the photoresistutilized has a negative profile. Various embodiments of the presentinvention, which are described below, are used to accelerate thelift-off process and increase the process margin for photoresist removalfollowing deposition of the dielectric optical coating. By acceleratingthe lift-off process and increasing the process margin, embodiments ofthe present invention provide for more inexpensive light sensors thatinclude dielectric optical coatings to shape their spectral response.

Certain embodiments of the present invention described with reference toFIG. 6, which includes FIGS. 6( a), (b), (c), (d), and (e), involve theuse of a short duration soft baking step (also referred to as a softbake process) that occurs after the deposition of the dielectric opticalcoating, but before performance of the lift-off step using a solvent.

Referring to FIG. 6( a), a photoresist 620 is deposited on aphotodetector sensor region 604 formed in a semiconductor substrate 606.Still referring to FIG. 6( a), a photomask 630 is used to selectivelyexpose a portion of the underlying structure to UV light, causingexposure of a portion of the photoresist to the UV light, while anotherportion of the photoresist is unexposed to the UV light. As shown inFIG. 6( b), a developer is then used to remove the photoresist 620 thatwas unexposed to UV light, without removing the photoresist that wasexposed to UV light. While the profile of the remaining photoresistshown in FIG. 6( b) is a vertical profile (which is also known as aneutral profile), the photoresist profile can alternatively be negativeor positive (as shown in FIGS. 4( b) and 5(b), respectively).

Thereafter, the dielectric optical coating 640 is deposited, to therebyform a dielectric optical filter over the light sensor, as shown in FIG.6( c). In accordance with an embodiment, the photoresist has a softeningtemperature (T_(soft), which is also known as the softening point) thatis higher than the temperature (T_(dep)) at which the deposition of thedielectric optical coating is performed. Stated another way, inaccordance with an embodiment, the optical coating deposition isperformed at a temperature (T_(dep)) below the softening point(T_(soft)) of the photoresist, i.e., T_(dep)<T_(soft). Most photoresistshave a softening point (T_(soft)) in the range of 100-130 degreesCelsius. At this point, the dielectric optical coating 640 covers thephotodetector sensor region 604, a circumferential area surrounding thesensor region 604, and the portion of the photoresist 620 that remainedafter the developer was used to remove the photoresist that wasunexposed to UV light.

In accordance with an embodiment, following the deposition of thedielectric optical coating 640, a short duration soft bake is performedat a temperature (T_(soft) _(—) _(bake)) above the softening point(T_(soft)) of the photoresist, i.e., T_(soft) _(—) _(bake)>T_(soft).While the duration of the soft bake is dependent on the thickness andtype of the photoresist, it is estimated that the soft bake durationshould be about 0.5 to 1 minute per micron (μm) thickness of thephotoresist. Since the typical photoresist thickness ranges from about 3to 6 μm, the soft bake process would likely take about 1.5 minutes toabout 6 minutes, but may take less or more time. Preferably, the softbake time does not exceed 15 minutes, and more preferably does notexceed 5 minutes. The soft bake causes thermal expansion of thedielectric optical coating and the photoresist, and also causesphotoresist reflow, which individually and collectively cause tensilemechanical strain at the sidewall of the dielectric optical coating,which in-turn causes micro-cracks to form in the dielectric opticalcoating. The photoresist reflow is generally shown at 650 in FIG. 6( d).The micro-cracks are generally illustrated by the dashed crooked line660 in FIG. 6( d). A lift-off process using a solvent is thereafterperformed to remove the remaining photoresist and the dielectric opticalcoating covering the photoresist, which results in the structure shownin FIG. 6( e). The micro-cracks 660 provide additional avenues for thelift-off solvent to reach the photoresist during the lift-off process,thereby significantly reducing the soak duration required for lift-off.

The use of the soft bake step should reduce the soak duration (duringthe lift-off process) by at least a factor of two, and potentially by afactor of ten or more. For example, it is estimated that the lift-offprocess will take about 15 to 30 minutes following the soft bake step,which is significantly faster than the up to nine hours it might take ifthe soft bake step was not performed.

FIG. 7, which includes FIGS. 7( a), (b), (c), (d), (e) and (f),illustrates in more detail how the photoresist reflow occurs during thea short duration soft bake process. More specifically, the photoresistin FIG. 7( a) is shown as having a vertical profile (also known as aneutral profile), similar to the photoresist profile shown in FIG. 6(b). As can be appreciated from FIGS. 7( b), (c), (d) and (e), thephotoresist profile begins to become rounded due to reflow, resulting inthe photoresist profile of FIG. 7( f) at the end of the short durationsoft bake process. In accordance with specific embodiments, the shortduration soft bake process occurs for about 15 minutes or less, andpreferably for about 5 minutes or less, and even more preferably forabout 2 minutes or less. In specific embodiments, the short durationsoft bake process occurs for between about 30 and 60 seconds.

While it is preferred that the soft bake is performed at a temperature(T_(soft) _(—) _(bake)) above the softening point temperature (T_(soft))of the photoresist, i.e., T_(soft) _(—) _(bake)>T_(soft), micro-crackswould likely also occur at temperatures just below the softening point(T_(soft)), e.g., within 10 degrees Celsius of the softening pointtemperature. It is also noted that the soft bake process is preferablynot performed at a temperature that is much greater than the softeningpoint temperature (T_(soft)), so at to prevent the photoresist fromliquefying.

While it is preferred that the soft bake is performed at a temperature(T_(soft) _(—) _(bake)) above the softening point temperature (T_(soft))of the photoresist, i.e., T_(soft) _(—) _(bake)>T_(soft), micro-cracksshould also occur at temperatures just below the softening pointtemperature (T_(soft)), e.g., within 10 degrees Celsius of the softeningpoint temperature. Accordingly, the short duration soft bake can be moregenerally be performed within a predetermined range of the softeningpoint (T_(soft)) of the photoresist to thereby form and/or increase anumber of micro-cracks in the dielectric optical coating that is notcovering the photodetector sensor region. In accordance with anembodiment, the predetermined range of the softening point temperature(T_(soft)) of the photoresist is +/−10 degrees Celsius of the softeningpoint temperature (T_(soft)).

Referring now to FIG. 8, the arrows 802 shown therein point to cornersof the photoresist where micro-cracks 860 in the dielectric opticalcoating originated due to the high mechanical stresses caused by thesoft bake process. The micro-cracks 860 illustrate the same type ofmicro-cracks 660 discussed above with reference to FIG. 6.

Further embodiments of the present invention, which will be describedwith reference to FIGS. 9 and 10, involve the use of thermal shock (alsoreferred to as temperature cycling) after the deposition of thedielectric optical coating, but before performance of the lift-off stepusing a solvent.

FIGS. 9( a), (b) and (c) of FIG. 9 are similar to FIGS. 6( a), (b) and(c) of FIG. 6, and thus, need not be described again. More specifically,in FIG. 9, the reference numerals 904, 906, 920, 930 and 940 representsubstantially the same elements as the reference numerals 604, 606, 620,630 and 640 in FIG. 6, and thus these elements need not be describedagain. Referring to FIG. 9( d) and FIG. 10, after the deposition of theoptical coating 940, thermo-mechanical stresses are introduced bycycling between two (or more) temperatures T1 and T2, where T1<<T2. Inspecific embodiments, the temperature T2 is between about 50 and 150degrees Celsius greater than the temperature T1. More specifically, thetemperature T1 is within the range of about 0 to 50 degrees Celsius, andthe temperature T2 is within the range of about 100 to 150 degreesCelsius, in accordance with an embodiment. Preferably, the temperatureT2 is between about 90 and 120 degrees Celsius greater than thetemperature T1, the temperature T1 is within the range of about 0 to 30degrees Celsius, and the temperature T2 is within the range of about 120to 150 degrees Celsius. Alternative temperatures may be used, dependingon the photoresist formulation.

In accordance with an embodiment, the temperature cycling is performedwithin a dry oven with inert atmosphere. In accordance with anembodiment, the transitions between T1 and T2, and between T2 and T1,are relatively rapid, as shown in FIG. 10, to promote stress anddelamination of the dielectric optical coating 940 over the photoresist920, as well as photoresist reflow, all of which contribute tomicro-cracks 960. In accordance with an embodiment, the temperaturecycling is performed for about 60 to 120 seconds, with the temperatureremaining at each of the different temperature (e.g., T1 and T2) for atleast 10 seconds, before transitioning to another one of thetemperatures. In accordance with specific embodiments, the temperaturecycling process occurs for about 15 minutes or less, and preferably forabout 5 minutes or less, and even more preferably for about 2 minutes orless. In specific embodiments, the temperature cycling process occursfor between about 1 and 2 minutes. Other variations are possible, andwithin the scope of an embodiment of the present invention.

Micro-cracks are caused by the temperature cycling, as a result ofthermal expansion and thermal contraction, and are generally illustratedby the dashed crooked line 960 in FIG. 9( d). The arrows 970 are used toillustrate delamination. Both the dielectric optical coating 940 and thephotoresist 920 will expand and contract during the temperature cycling.However, photoresist polymers have a much higher coefficient ofexpansion (typically at least 10× higher) than dielectric opticalcoatings, which advantageously contributes to the micro-cracking anddelaminating of the dielectric optical coating 960.

The use of the temperature cycling should reduce the soak duration(during the lift-off process) by at least a factor of two, and bypotentially by a factor of ten or more. For example, it is estimatedthat the lift-off process will take about 15 to 30 minutes following thetemperature cycling step, which is significantly faster than the up tonine hours it might take if the temperature cycling step was notperformed.

While transitioning between two temperatures (e.g., T1 and T2) issufficient, it is possible and within the scope of an embodiment of thepresent invention that three or more different temperatures (e.g., T1,T2 and T3) can be used to perform the thermal shock.

In accordance with certain embodiments, the soft bake process can beperformed for a first period of time, and then the temperature cyclingprocess can be performed for a second period of time, prior to using achemical solvent bath to complete the lift-off process. In accordancewith other embodiments, the temperature cycling process can be performedfor a first period of time, and then the soft-bake process can beperformed for a second period of time, prior to using a chemical solventbath to complete the lift-off process. In other words, theaforementioned soft-bake and temperature cycling embodiments can both beused, e.g., one after the other.

Referring now to FIG. 11, illustrated therein is a top down view of anexemplary light sensor in the process of being manufactured. Morespecifically, FIG. 11 corresponds to the point in the fabricationprocess after which the following steps have already been performed. Asurface of a semiconductor substrate, which includes a rectangularphotodetector sensor region 1104, has been covered with a photoresist.Photolithography has been used to expose a portion of the photoresistwhile not exposing a portion of the photoresist covering thephotodetector sensor region 1104. Additionally, a circumferential area1110 that extends beyond the sensor region 1104, having a width “d”, wasalso not exposed. A developer has been used to remove the portion of thephotoresist covering the photodetector sensor region 1104, as well as toremove the portion of the photoresist covering the circumferential area1110, leaving the portion of the photoresist 1120 that was exposedduring photolithography (which can be referred to as the exposed orremaining photoresist 1120). Also, a dielectric optical coating has beendeposited over the sensor region 1104, over the circumferential area1110, and over the remaining photoresist 1120. Accordingly, FIG. 11essentially corresponds to a top down view of what is shown in FIGS. 4(c) and 5(c) discussed above.

Still referring to FIG. 11, the sensor region 1104 is shown as having arectangular outer perimeter. The photoresist 1120 (which remained afterthe developer had been used to remove the portion of the photoresistcovering the photodetector sensor region 1104 and the circumferentialarea 1110) has a rectangular opening that is similar in shape to therectangular outer perimeter of the sensor region 1104. This rectangularshaped opening in the photoresist 1120 is achieved using a rectangularfeature in the photomask during the photolithography step that exposes aportion of the photoresist while not exposing a portion of thephotoresist covering the sensor region 1104 and the circumferential area1110. The next step of the fabrication process would be to perform alift-off process using a solvent to remove the remaining photoresist1120 and the portion of the dielectric optical coating covering thephotoresist 1120, to thereby produces a structure similar to what wasshown in FIGS. 4( d) and 5(d). However, as mentioned above, a problemwith conventional lift-off techniques is that following the dielectricoptical coating deposition, the lift-off process typically requires avery long soak duration (on the order of multiple hours) due to thephotoresist not being fully exposed to the solvent (because thephotoresist is covered by the dielectric optical coating).

FIGS. 12( a), 12(b) and 12(c) are perspective cross-sectional views thatcorrespond to a portion of the light sensor shown in FIG. 11, duringdifferent steps of the fabrication process. FIG. 12( a) shows a portionof a straight sidewall of the photoresist (over the sensor substrate)following exposure to a developer. FIG. 12( b) shows the same portionafter deposition of the dielectric optical coating. FIG. 12( c)illustrates how the dielectric optical coating remains and thephotoresist is removed as a result of the liftoff process.

The embodiments of the present invention described above with referenceto FIGS. 6-10 relied on a soft bake process and/or a temperature cyclingprocess to purposely form micro-cracks in (and potentially causedelaminating of) the dielectric optical coating covering thephotoresist, which enable the lift-off process to be performed much morequickly than if no (or less) micro-cracks were formed. Additionalembodiments of the present invention, which are described below, usealternative techniques to purposely form micro-cracks in the dielectricoptical coating covering the photoresist, and more specifically, tosignificantly increase the number of micro-cracks. Thus, such additionalembodiments can also be used to significantly reduced the amount of timerequired for the lift-off process.

The inventors have discovered that micro-cracks more readily appear nearsharp corners in the photoresist over which the dielectric opticalcoating is deposited. In FIG. 11, note that the rectangular opening inthe photoresist 1120 includes only four sharp corners. To increase thenumber of micro-cracks formed in the dielectric optical coating coveringthe photoresist, one or more dummy corners, dummy islands and/or dummyrings are included in the photomask used during the photolithographystep (to expose a portion of the photoresist while not exposing aportion of the photoresist covering the photodetector sensor region).Such added corners, islands and rings are referred to as “dummy”features because they are not actually needed to form an underlyingfeature of the light sensor being manufactured, but rather, are includedfor the sole purpose of increasing micro-crack formations as well as theedge surface area, thereby improving lift-off efficiency andconsistency. More specifically, the additional corners increase localstrain, which results in micro-cracks that are similar to thosedescribed above with reference to FIGS. 6( d) and 9(d). The additionaledge area and micro-cracks both accelerate the photoresist removal. Aswill be appreciated from FIGS. 14, 16 and 19, discussed below, featurescorresponding to the one or more dummy corners, dummy islands and/ordummy rings will remains following the photoresist removal step.However, because these remaining features (which can be referred to asdummy features) are outside the active photodetector sensor region, theydo not adversely affect operation of the final light sensor.

The various embodiments of the present invention, described below withreference to FIGS. 13-19, involve the adding of dummy corners, dummyislands and/or dummy rings (all of which add additional corners) in thephotomask, which result in additional corners and edge surface area atthe perimeters of the photoresist and dielectric optical coating.Referring to FIG. 13, illustrated therein is a top down view ofphotodetector sensor region 1304 having a rectangular perimeter, as wasthe case also in FIG. 11. However, in contrast to FIG. 11, in FIG. 13the perimeters of the photoresist and dielectric optical coatingperimeter have numerous additional 90 degree corners 1350. Theseadditional corners 1350, which are examples of dummy corners, areachieved by adding dummy corners in the photomask used to exposeselective portions of the photoresist to UV light. The added cornersalso increase the edge surface areas (which can also be referred to asthe perimeter surface areas) of the photoresist and the dielectricoptical coating by about 40% compared to the edge surface areas in FIG.11. The dummy corners increase local strain, which results in anincreased number of micro-cracks that are similar to those describedabove with reference to FIGS. 6( d), 8 and 9(d). The additional edgesurface areas and the additional micro-cracks both accelerate thephotoresist removal and increase the lift-off process margin. In FIG. 13the perimeters of the photoresist and dielectric optical coatingincludes a total of forty-four corners, forty of which can be considereddummy corners.

FIGS. 14( a), 14(b) and 14(c) are perspective cross-sectional views thatcorrespond to a portion of the light sensor shown in FIG. 13, duringdifferent steps of the fabrication process. FIG. 14( a) shows a portionof a sidewall of the photoresist (over the sensor substrate) followingexposure to a developer, wherein the sidewall includes four more 90degree corners compared to FIG. 12( a). FIG. 14( b) shows the sameportion after deposition of the dielectric optical coating, similarlyshowing four more 90 degree corners compared to FIG. 12( b). FIG. 14( c)illustrates how the dielectric optical coating remains and thephotoresist is removed as a result of the liftoff process. The portionof the dielectric optical coating that is shown in FIG. 14( c), but wasnot shown in FIG. 12( c), can be considered a dummy dielectric opticalcoating feature. However, because the dummy dielectric optical coatingfeature is outside of the photodetector sensor region, it will notadversely affect the function of the resulting light sensor.

Referring now to FIG. 15, illustrated therein is a top down view of aphotodetector sensor region 1504 having a rectangular perimeter, as wasthe case also in FIGS. 11 and 13. Also shown are the circumferentialarea 1510 covered by the dielectric optical coating (with no underlyingphotodetector sensor region and no underlying photoresist), and theexposed photoresist 1520 covered by the dielectric optical coating. Incontrast to FIG. 11, in FIG. 15 the photoresist 1520 has severalrectangular dummy islands 1560 (ten are shown) beyond the sensor region1504. Each rectangular dummy island 1560, which can also be referred toas a dummy island opening in the photoresist, include four 90 degreecorners 1550. These additional corners 1550, which are also examples ofdummy corners, increase local strain, which results in an increasednumber of micro-cracks that are similar to those described above withreference to FIGS. 6( d), 8 and 9(d). The additional edge surface areaand the additional micro-cracks both accelerate the photoresist removaland increase the lift-off process margin. In FIG. 15, since each of theten dummy islands 1560 adds four dummy corners 1550, there are a totalof forty dummy corners 1550.

FIGS. 16( a), 16(b) and 16(c) are perspective cross-sectional views thatcorrespond to a portion of the light sensor shown in FIG. 15, duringdifferent steps of the fabrication process. FIG. 15( a) shows a portionof a sidewall of the photoresist (over the sensor substrate) followingexposure to a developer, wherein the photoresist layer includes arectangular opening or window having four 90 degree corners. FIG. 16( b)shows the same portion after deposition of the dielectric opticalcoating. FIG. 16( c) illustrates how the dielectric optical coatingremains and the photoresist is removed as a result of the lift-offprocess. The portion of the dielectric optical coating that is shown inFIG. 16( c), but was not shown in FIG. 12( c), can be considered a dummydielectric optical coating feature. However, because the dummydielectric optical coating feature is outside of the photodetectorsensor region, it will not adversely affect the function of theresulting light sensor.

FIG. 17 illustrate an embodiment which essentially combines theembodiment described with reference to FIGS. 13 and 14 with theembodiment described with reference to FIGS. 15 and 16. FIG. 17 shows aphotodetector sensor region 1704 having a rectangular perimeter, as wasthe case also in FIGS. 11, 13 and 15. Also shown are the circumferentialarea 1710 covered by the dielectric optical coating (with no underlyingphotodetector sensor region and no underlying photoresist) and theexposed photoresist 1720 covered by the dielectric optical coating. InFIG. 17, there are fourteen dummy islands 1760, each of which providefour dummy corners 1750. Also, there are an additional forty dummycorners 1750 not provided by the dummy islands. Thus, in FIG. 17, thereare a total of ninety-six dummy corners 1750.

FIGS. 18 and 19 illustrate yet another variation on the embodimentsdescribed above with reference FIGS. 13-17. FIG. 18 shows aphotodetector sensor region 1804 having a rectangular perimeter, as wasthe case also in FIGS. 11, 13, 15 and 17. Also shown are thecircumferential area 1810 covered by the dielectric optical coating(with no underlying photodetector sensor region and no underlyingphotoresist) and the exposed photoresist 1820 covered by the dielectricoptical coating. A dummy ring 1870 is provided, which adds eight dummycorners 1850. Additionally, the dummy ring 1870 increases the edgesurface areas of the photoresist and the dielectric optical coating byabout 100% compared to the edge surface areas in FIG. 11. FIGS. 19( a),19(b) and 19(c) are perspective cross-sectional views that correspond tothe light sensor shown in FIG. 18, during different steps of thefabrication process.

While the dummy corners shown in FIGS. 13-19 were shown as being 90degree corners, dummy corners of other angles are also possible andwithin the scope of the present invention. Further, it is noted thateach of the dummy corners is preferably a sharp corner, as opposed to arounded corner, because sharp corners result in more local stress andthereby will result in more micro-cracks.

The embodiments described with reference to FIGS. 13-19 can be combinedwith the embodiments described with reference to FIGS. 6-8 and/or theembodiments described with reference to FIGS. 8-9.

For illustrative purposes, exemplary additional details of a dielectricoptical coating (which can also be referred to as a dielectric opticalfilter) are provided below. The dielectric materials used to form adielectric optical filter can include silicon dioxide (SiO2), siliconhydride (SixHy), silicon nitride (SixNy), silicon oxynitride (SixOzNy),tantalum oxide (TaxOy), gallium arsenide (GaAs), gallium nitride (GaN),and the like. Alternating layers in the optical filter may have aconstant or varying film thickness throughout the filter stack, in orderto achieve the desired optical response. By careful choice of the exactcomposition, thickness, and number of these layers, it is possible totailor the reflectivity and transmissivity of the optical filter toproduce almost any desired spectral characteristics. For example, thereflectivity can be increased to greater than 99.99%, to produce ahigh-reflector (HR) coating. The level of reflectivity can also be tunedto any particular value, for instance to produce a mirror that reflects90% and transmits 10% of the light that falls on it, over some range ofwavelengths. Such mirrors have often been used as beam splitters, and asoutput couplers in lasers. Alternatively, the optical filter can bedesigned such that the mirror reflects light only in a narrow band ofwavelengths, producing a reflective optical filter.

Generally, layers of high and low refractive index materials arealternated one above the other. This periodic or alternating structuresignificantly enhances the reflectivity of the surface in the certainwavelength range called band-stop, which width is determined by theratio of the two used indices only (for quarter-wave system), while themaximum reflectivity is increasing nearly up to 100% with a number oflayers in the stack. The thicknesses of the layers are generallyquarter-wave (then they yield to the broadest high reflection band incomparison to the non-quarter-wave systems composed from the samematerials), designed such that reflected beams constructively interferewith one another to maximize reflection and minimize transmission. Usingthe above described structures, high reflective coatings can achievevery high (e.g., 99.9%) reflectivity over a broad wavelength range (tensof nanometers in the visible spectrum range), with a lower reflectivityover other wavelength ranges, to thereby achieve a desired spectralresponse. By manipulating the exact thickness and composition of thelayers in the reflective stack, the reflection characteristics can betuned to a desired spectral response, and may incorporate bothhigh-reflective and anti-reflective wavelength regions. The coating canbe designed as a long-pass or short-pass filter, a bandpass or notchfilter, or a mirror with a specific reflectivity.

In accordance with specific embodiments of the present invention, anoptical filter is used to shape the spectral response of the underlyingphoto detector region to obtain a true human eye spectral response,i.e., a response that is similar to that of a typical human eyeresponse. Alternative spectral responses are possible, and within thescope of the present invention.

Various embodiments of the present invention will now be summarized withreference to the high level flow diagrams of FIGS. 20-22.

FIG. 20 is used to summarize the short duration soft bake embodimentsdescribed above with reference to FIGS. 6-8. Referring to FIG. 20, atstep 2002, a surface of a semiconductor substrate, which includes aphotodetector sensor region, is covered with a photoresist having asoftening point (T_(soft)). At step 2004, photolithography is used toexpose a portion of the photoresist while not exposing a portion of thephotoresist covering the photodetector sensor region. At step 2006, theportion of the photoresist covering the photodetector sensor region isremoved using a developer. At step 2008, a dielectric optical coating isdeposited over the photodetector sensor region and over the photoresistnot covering the photodetector sensor region. At step 2010, a shortduration soft bake at a temperature (T_(soft) _(—) _(bake)) is performedwithin a predetermined range of the softening point (T_(soft)) of thephotoresist to thereby form and/or increase a number of micro-cracks inthe dielectric optical coating not covering the photodetector sensorregion. At step 2012, the dielectric optical coating that is over thephotoresist not covering the photodetector sensor region is lifted off,so that the resulting light sensor includes the dielectric opticalcoating over the photodetector sensor region. Additional details of thetechniques summarized in FIG. 20 can be appreciated from the abovediscussion of FIGS. 6-8.

FIG. 21 is used to summarize the temperature cycling embodimentsdescribed above with reference to FIGS. 9-10. Referring to FIG. 21,steps 2102, 2104, 2106 and 2108 of FIG. 21 are identical to steps 2002,2004, 2006 and 2008 described above with reference to FIG. 20, and thusneed not be described again. At step 2110, temperature cycling isperformed by repetitively cycling back and forth between at least twotemperatures T1 and T2, to thereby form and/or increase a number ofmicro-cracks in the dielectric optical coating not covering thephotodetector sensor region. In specific embodiment, the temperature T2is between about 50 and 120 degrees Celsius greater than the temperatureT1. At step 2112, the dielectric optical coating that is over thephotoresist not covering the photodetector sensor region is lifted off,so that the resulting light sensor includes the dielectric opticalcoating over the photodetector sensor region. Additional details of thetechniques summarized in FIG. 21 can be appreciated from the abovediscussion of FIGS. 9-10.

FIG. 22 is used to summarize the embodiments described above withreference to FIGS. 11-19. Referring to FIG. 22, at step 2202, a surfaceof a semiconductor substrate, which includes a photodetector sensorregion, is covered with photoresist. At step 2204, photolithography isperformed using a photomask to expose a portion of the photoresist,while not exposing a portion of the photoresist covering thephotodetector sensor region. As was described above with reference toFIGS. 13, 15, 17 and 18, the photomask used at step 2204 includes one ormore dummy corners, dummy islands and/or dummy rings. At step 2206, theportion of the photoresist covering the photodetector sensor region isremoved using a developer. At step 2208, a dielectric optical coating isdeposited over the photodetector sensor region and the portion of thephotoresist that remains following the removing at step 2206. At step2210, the dielectric optical coating that is over the photoresist notcovering the photodetector sensor region is removed, so that theresulting light sensor includes the dielectric optical coating over thephotodetector sensor region. Additional details of the techniquessummarized in FIG. 22 can be appreciated from the above discussion ofFIGS. 11-19.

Packaged optical sensors are typically subject to significant thermalstresses during accelerated reliability testing, which can include,temperature cycling, moisture sensitivity level (MSL) tests, highlyaccelerated stress tests (HAST), etc. These stresses arise from thethermal expansion and contraction of films underneath the dielectricoptical coating on the die substrate, as well as the thermal expansionand contraction of the package encapsulation material (which istypically a transparent epoxy). Such stresses can cause the dielectricoptical coating (that covers and extends beyond the photodetector sensorregion) to crack and/or delaminated, which reduces yield.

In accordance with specific embodiments of the present invention,non-orthogonal (e.g., 45°, but not limited thereto) chamfered cornersare added to improve the thermal reliability of the dielectric opticalcoating the covers the photodetector sensor region. Such embodiments canbe appreciated from FIGS. 23 and 24, which are described below.

Referring first to FIG. 23, this figure is the substantially the same asFIG. 15, except that chamfered corners 2380 are added to the dielectricoptical coating to improve the thermal reliability of the dielectricoptical coating over a photodetector sensor region 2304. Morespecifically, FIG. 23 illustrates a top down view of a photodetectorsensor region 2304 having a rectangular perimeter. Also shown are thecircumferential area 2310 covered by the dielectric optical coating(with no underlying photodetector sensor region and no underlyingphotoresist), and the exposed photoresist 2320 covered by the dielectricoptical coating. As was the case in FIG. 15, the photoresist 2320 hasrectangular dummy islands 2360 beyond the sensor region 2304, which addsdummy corners 2350. The chamfered corners 2380 can be achieved byincluding chamfered corners in the photomask that is used during thephotolithography step that exposes a portion of the photoresist whilenot exposing a portion of the photoresist covering the photodetectorsensor region 2304 and the circumferential area 2310.

Referring next to FIG. 24, this figure is substantially the same as FIG.18, excepts that chamfered corners 2480 are added to the dielectricoptical coating to improve the thermal reliability of the dielectricoptical coating over a photodetector sensor region 2404. Morespecifically, FIG. 24 illustrates a top down view of a photodetectorsensor region 2404 having a rectangular perimeter, a circumferentialarea 2410 covered by the dielectric optical coating (with no underlyingphotodetector sensor region and no underlying photoresist), exposedphotoresist 2420 covered by the dielectric optical coating, and a dummyring 2470 that adds eight dummy corners 2450.

FIGS. 23 and 24 illustrates how chamfered corners can be included in theembodiments described above with reference to FIGS. 11-19 and 22. Thechamfered corners can also be included in the embodiments describedabove with reference to FIGS. 6-10 and 21. It is also within the scopeof an embodiment of the present invention that the chamfered corners notbe combined with the other embodiments described herein.

FIG. 25 is used to summarize the embodiments described above withreference to FIGS. 23 and 24. Referring to FIG. 25, at step 2502, asurface of a semiconductor substrate, which includes a photodetectorsensor region, is covered with photoresist. At step 2504,photolithography is performed using a photomask to expose a portion ofthe photoresist, while not exposing a portion of the photoresistcovering the photodetector sensor region. As was described above withreference to FIGS. 23 and 24, a portion of the photomask used to notexpose the portion of the photoresist covering the photodetector sensorregion includes chamfered corners. At step 2506, the portion of thephotoresist covering the photodetector sensor region is removed using adeveloper. At step 2508, a dielectric optical coating is deposited overthe photodetector sensor region and the portion of the photoresist thatremains following the removing at step 2506. At step 2510, thedielectric optical coating that is over the photoresist not covering thephotodetector sensor region is removed, so that the resulting lightsensor includes the dielectric optical coating over the photodetectorsensor region. Because of the chamfered corners included in photomaskused at step 2504, following the lifting off at step 2510, a portion ofthe dielectric optical coating that extends beyond the photodetectorsensor region includes chamfered corners. Additional details of thetechniques summarized in FIG. 25 can be appreciated from the abovediscussion of FIGS. 23 and 24.

There exist both positive and negative types of photoresists. When apositive photoresist is exposed to UV light the chemical structure ofthe photoresist changes so that it becomes more soluble in a developer.The exposed photoresist is then washed away by the developer, leavingwindows in the photoresist where the photoresist was exposed to UVlight. Accordingly, when using a positive photoresist the photomaskincludes an exact copy of the pattern which is to remain on the wafer.Negative photoresists behave in the opposite manner. That is, exposureto the UV light causes the negative photoresist to become less solublein a developer. Therefore, the negative photoresist remains on thesurface wherever it was exposed, and the developer removes only theunexposed portions. Accordingly, a photomask used with a negativephotoresist includes the inverse (or photographic “negative”) of thepattern to be transferred.

In the embodiments described above, the photoresists (e.g., 620, 920,1120, 1230, 1520, 1720, 1820, 2320 and 2420) that were described behavedas, and thus were, negative photoresists. However, it is also within thescope of embodiments of the present invention to use positivephotoresists instead of negative photoresists. Accordingly, steps 2004and 2104 (in FIGS. 20 and 21, respectively) can more generally involvedefining a pattern in the photoresist. Similarly, step 2204 (in FIG. 22)can more generally involve performing photolithography using a photomaskto define a pattern in the photoresist that includes one or more dummycorners, dummy islands and/or dummy rings. Further, step 2504 (in FIG.25) can more generally involve performing photolithography using aphotomask to define a pattern in the photoresist that includes chamferedcorners.

Embodiments of the present invention are also directed to light sensorsformed used the above described techniques, and systems that includesuch sensors.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiments shown. Therefore, it ismanifestly intended that this invention be limited only by the claimsand the equivalents thereof.

What is claimed:
 1. A method for fabricating a light sensor, comprising:(a) covering a surface of a semiconductor substrate, which includes aphotodetector sensor region, with photoresist having a softening point(T_(soft)); (b) using photolithography to define a pattern in thephotoresist; (c) using a developer, removing a portion of thephotoresist that covered the photodetector sensor region and covered acircumferential region surrounding the photodetector sensor region; (d)depositing a dielectric optical coating over the photodetector sensorregion, over the circumferential region, and over the photoresist notcovering the photodetector sensor region that remained after theremoving at step (c); (e) performing a short duration soft bake at atemperature (T_(soft) _(—) _(bake)) within a predetermined range of thesoftening point (T_(soft)) of the photoresist to thereby form and/orincrease a number of micro-cracks in the dielectric optical coating notcovering the photodetector sensor region; and (f) lifting off a portionof the dielectric optical coating that is over the remaining photoresistnot covering the photodetector sensor region, so that a remainingportion of the dielectric optical coating covers the photodetectorsensor region and the circumferential region.
 2. The method of claim 1,wherein the short duration soft bake performed at step (e) is performedfor 15 minutes or less.
 3. The method of claim 1, wherein the shortduration soft bake performed at step (e) is performed for between about30 and 60 seconds.
 4. The method of claim 1, wherein the predeterminedrange of the softening point (T_(soft)) of the photoresist is +/−10degrees Celsius of the softening point (T_(soft)).
 5. The method ofclaim 1, wherein the predetermined range of the softening point(T_(soft)) of the photoresist includes the softening point (T_(soft))and temperatures that are up to 10 degrees Celsius greater than thesoftening point (T_(soft)), inclusive.
 6. The method of claim 1, whereinstep (f) comprises performing a chemical solvent soak for no more thanone hour.
 7. The method of claim 1, wherein step (f) comprisesperforming a chemical solvent soak for between about 15 to 30 minutes.8. A method for fabricating a light sensor, comprising: (a) covering asurface of a semiconductor substrate, which includes a photodetectorsensor region; (b) using photolithography to define a pattern in thephotoresist; (c) using a developer, removing a portion of thephotoresist that covered the photodetector sensor region and covered acircumferential region surrounding the photodetector sensor region; (d)depositing a dielectric optical coating over the photodetector sensorregion, over the circumferential region, and over the photoresist notcovering the photodetector sensor region that remained after theremoving at step (c); (e) repetitively cycling back and forth between atleast two temperatures T1 and T2, where the temperature T2 is betweenabout 50 and 150 degrees Celsius greater than the temperature T1, tothereby form and/or increase a number of micro-cracks in the dielectricoptical coating not covering the photodetector sensor region; and (f)lifting off a portion of the dielectric optical coating that is over theremaining photoresist not covering the photodetector sensor region, sothat a remaining portion of the dielectric optical coating covers thephotodetector sensor region and the circumferential region.
 9. Themethod of claim 8, wherein: the temperature T1 is within the range ofabout 0 to 50 degrees Celsius; and the temperature T2 is within therange of about 100 to 150 degrees Celsius.
 10. The method of claim 8,wherein the temperature T2 is at least 90 degrees Celsius greater thanthe temperature T2.
 11. The method of claim 8, wherein the temperatureT2 between about 90 and 120 degrees Celsius greater than the temperatureT1.
 12. The method of claim 8, wherein: step (e) is performed within anoven; and during the repetitively cycling back and forth between the atleast two temperatures T1 and T2 at step (e), the temperature of theoven remains at one of the two temperatures T1 and T2 for at least 10seconds before transitioning to another one of the temperatures T1 andT2.
 13. The method of claim 8, wherein the repetitively cycling back andforth between the at least two temperatures T1 and T2 at step (e) isperformed for 15 minutes or less.
 14. The method of claim 8, wherein therepetitively cycling back and forth between the at least twotemperatures T1 and T2 at step (e) is performed for between about 1 and2 minutes.
 15. The method of claim 8, wherein step (f) comprisesperforming a chemical solvent soak for no more than one hour.
 16. Themethod of claim 8, wherein step (f) comprises performing a chemicalsolvent soak for between about 15 to 30 minutes.